The invention relates generally to nuclear magnetic resonance imaging (“MRI”), and more particularly to a technique for using MRI to determine the location of a device being used in a surgical or medical procedure.
MRI systems have become ubiquitous in the field of medical diagnostics. In general, MRI systems are based on the interactions among a primary magnetic field, a radiofrequency (rf) field and time varying magnetic gradient fields with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to obtain rf signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of three gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radiofrequency coil is employed to produce an rf field, typically pulsed to create the required resonance signals. This rf magnetic field perturbs the spin system from its equilibrium direction, causing the spins to process at desired phases and frequencies. During this precession, rf fields are emitted by the affected molecules and detected by either the same transmitting rf coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Heretofore, MRI systems have also been employed to determine the location of a device (such as a catheter) during medical or surgical procedures. Such MR tracking systems employ small tracking coils attached to the device to be tracked. During these MR tracking procedures, signals are generated throughout the patient using the large transmitting rf coil, but are detected with the small tracking coils attached to the device. Locating the tracking coils is typically accomplished by acquiring the MR signal in the presence of the applied magnetic field gradient, Fourier transforming the signal, and identifying the position of the most intense frequency-domain signal.
Frequently, these tracking coils are almost fully immersed in MR signal generating fluids. This is particularly true for MR tracking catheters. Because the tracking coils detect signals from their entire surroundings, localization of the MR signal can be difficult when the data pixel size is smaller than that of the coil. Localization becomes even more difficult when the signal-to-noise ratio (SNR) is relatively low. Under these conditions the measured location of the tracking coil appears to hop around the true location of the coil since the local signal maximum varies both spatially and temporally.
One way to improve the precision of the location measurement is to increase the SNR of the acquisition. This can be done by 1) increasing the static magnetic field strength, 2) signal averaging, 3) using larger tracking coils and/or 4) changing the T1 of the MR signal source. Unfortunately, all of these remedies have implications for system cost, resolution (temporal and spatial), and clinical use.
Accordingly, there is a need for an improved technique for employing device tracking with an MRI system. Particularly, there is a need for a technique that determines device location while addressing the undesirable effects of poor signal quality.